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178                          METALLUKGY OF IRON AND STEEL.
same in the bee-hive and the retort, the latter, however, giving an excess of gas for other uses; while with poor coals the yield of coke is greater in .the retort oven. It is not correct to say that the yield of coke can be accurately estimated from the laboratory tests on fixed carbon, for there is a complicated reaction in the retort oven and in the bee-hive, whereby the dense hydrocarbons are broken up after they are distilled and deposit carbon in the mass of coal, so that it is possible to produce more coke than there was fixed carbon in the coal. The proportion so made depends upon the molecular arrangement of each particular coal.
England has been slow in building retort ovens. They have been used for many years on the lean coals of South Wales, but it is only recently that they have come into general use in the Cleveland district and around Leeds. Rapid progress has been made within a few years. The total coke production of England is supposed to be from twelve to thirteen million tons; and the retort ovens now erected in the Kingdom have about one-quarter of that capacity.
SEC. IXf.ŚCoal washing.ŚMany deposits of coal contain a high percentage of ash or sulphur, or both. Proper washing will reduce both, but the extent of the purification will depend on the way in which they are combined. If most of the sulphur is in coarse grains of pyrite, it can be easily removed by a bumping-table or a one-spigot washer, but if a large proportion is fine, then some combination of sieves and jigs must be installed. If, unfortunately, the sulphur exists as sulphate of alumina or lime, or as organic matter, it may be impracticable, even by a jigging plant, to bring it down to the point required fox good coke. The washing of coal is a separation of minerals founded on their unlike rate of falling in water, but, under favorable conditions, the results obtained by very simple apparatus may suffice for commercial work. In many other cases more complicated processes are necessary, while in all cases the better apparatus will give a purer product. In one complete plant in Western Germany the coal in its natural state carries from 22 to 30 per cent, of ash, yhlch is reduced to about 10 per cent. At an English works a coal of 30 per cent, is brought down to 6 per cent. In Alabama 17.69 per cent, asli is cleaned down to 6.7 per cent., and 1.6 per cent, sulphur to 0.7* per cent.